(401u) A Systematic Investigation of Ionic Liquids As Effective Draw Solutes for Forward Osmosis

Authors: 
Limjuco, L. A., Myongji University
Zeweldi, H. G., Myongji University
Kim, H., Myongji University
Chung, W. J., Myongji University
Nisola, G. M., Myongji University
The growing concern on energy shortage has driven research to find energy-efficient technologies including those which mitigate water scarcity and environmental pollution. Among the alternative technologies, forward osmosis (FO) has emerged as one of the most promising candidates due to its versatile application in power generation, wastewater reclamation, and clean water production. This led to intensive research about FO which has an advantage of not using high hydraulic pressure as compared to reverse osmosis. Two elements of FO system are of importance to realize its feasibility as a practical and merchantable technology: FO membranes and draw solutes (DS). Despite the progress in membrane technology, draw solutions are still limited in terms of its reusability thereby hampering its economic viability.

In this study, fourteen ionic liquids (ILs) were investigated as DS for FO. These materials are coined as green substances due to their high ionicity, low vapor pressure and high thermal stability. Advancement in IL research has led to the development of polymerizable ILs or ILs incorporated in hybrid materials. All of these known properties of ILs suggest their strong potential as precursors for the development of next generation DS solutes. All IL candidates were thoroughly characterized and certain constants unavailable in literature were gathered. Initially, their Van’t Hoff factors (i) were determined by freezing point depression method. Their ability to generate osmotic pressures (p) were calculated and compared with NaCl as a conventional osmotic agent. The ILs were ranked and screened; their ability to generate osmotic pressures were correlated well with their ionicity (Van’t hoff), volume properties, melting points and diffusion coefficients. Ionic liquids which exhibited the highest Π were further investigated in FO operation conducted under FO and PRO modes. Results reveal the potential of ILs as alternatives to NaCl, especially under PRO mode wherein the internal dilutative concentration polarization (DICP) is much less than in FO mode. One significant benefit of ILs over NaCl is their negligible solute reverse flux and higher reverse flux selectivity. However, it is noted that some ILs were unsuitable given their high affinity to the selective layer of the CTA-FO membrane used. These results suggest that IL-membrane affinity played a critical role in the potential of certain ILs as DS. Thus, the best ILs are those which generate high osmotic pressures but have low affinity to the selective layer of the FO membrane. Suitable ILs can be recovered and recycled through membrane distillation (MD). A final hybrid system of FO-MD can be used for water purification and desalination application using ILs as DS.

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1A2B1009221) and the Ministry of Education (2009-0093816).